The present invention relates to a new method for producing Cuxe2x80x94Al2O3 composite powder having particle sizes less than 20 nm by thermo-chemical process. Its technology comprised in (1) the production of precursor powders by spray drying a water base solution made of water-soluble Cu and Al nitrates, (2) the heat treatment processing (desalting processing) in air atmosphere to remove the volatile components such as the moisture and NO3 group, and simultaneously to synthesize the nanophase CuOxe2x80x94Al2O3 composite powders and (3) the reduction processing of CuO to Cu to produce the nanophase Cuxe2x80x94Al2O3 composite powder.
It is well known that if the alumina particles are extremely fine and homogenous in the dispersion-strengthened Cu matrix-Al2O3 dispersed composite, the material strengths at room temperature can be still maintained up to the near melting point of copper due to the excellent dispersion-strengthening effect. And the loss of matrix electric conductivity is not also significant because the total alumina volume fraction can be minimized in favor of the nano particle size.
Consequently, based on the aforementioned characteristics of Cuxe2x80x94Al2O3 composites, these materials are extensively used as the high strength electric materials such as electrode for spot welding, lead wires, relay blades, and contact supports, and so on.
Especially, in the case of electrode materials used as the assembly for spot welder to join the body of the large vehicles, trains or robots, it requires superior strength at a high temperature and arcwear resistance for electrical discharge.
Neither the casting process nor the conventional powder metallurgy (P/M) process are suitable to produce such materials, because these processes always result in the possibility of segregation of hard particles and the limitation to produce or to distribute to ultra fine particles uniformly into meta matrix. Consequently these components should be manufactured by the P/M process through the preparation of composite powder containing the nano alumina particles.
Conventionally the nanophase Cuxe2x80x94Al2O3 composite powder has been produced by internal oxidation process. This method consists of following steps: (1) gas atomizing process to produce the Cu/Al alloy powder, (2) heat treatment in air or oxygen contained atmosphere for internal oxidation to produce CuO and Al2O3 composite powder, and (3) production of Cuxe2x80x94Al2O3 composite powder by means of the reducing CuO to Cu in hydrogen atmosphere.
The particle size of Al2O3 in the composite powder produced by means of the aforementioned internal oxidation has been known to 50 xcexcm in average. And oxygen during internal oxidation is diffused along the coarse Cu grain boundary. It can result in a segregation of oxide phase in grain boundary, moreover such non-uniformed hard phase can affect negatively the mechanical properties of the bulk materials produced by extrusion process.
Accordingly, in solving the aforementioned problems, the technical objective of the present invention lies in producing nanophase Cuxe2x80x94Al2O3 composite powder having homogenous distribution of oxide particles.
Therefore the method of the present invention comprises as follows:
1. The process of preparing a water base solution by dissolving the water-soluble salts of Cu-Nitrate (Cu(NO3)23H2O) and Al-Nitrate (Al(NO3)39H2O) for spray drying process.
2. The spray drying of prepared water base solution to produce a precursor powder consisting of a molecular scale mixture of Cu, Al, NO3xe2x80x94groups and moisture.
3. The process of desalting heat treatment in air atmosphere to remove the volatile components such as the moisture and NO3 and simultaneously to synthesize the nanophase CuOxe2x80x94Al2O3 composite powders.
The process of the heat treatment in hydrogen atmosphere to reduce the copper oxide to pure copper and finally to produce the nanophase CuO3xe2x80x94Al2O3 composite powders.